Highly corrosion resistant maraging stainless steel

ABSTRACT

A MARAGING STAINLESS STEEL CONTAINING CORRELATED AMOUNTS OF NICKEL, CHROMIUM, COBALT, SILICON AND METAL FROM THE GROUP CONSISTING OF ALUMINUM AND TITANIUM, THE STEEL BEING PARTICULARLY SUITABLE IN CAST FORM SINCE IT AFFORDS A COMBINATION OF GOOD STRENGTH, TOUGHNESS AND HIGH CORROSION RESISTANCE AND ALSO EXHIBITS EXCELLENT FOUNDRY CHARACTERISTICS.

United States Patent 3,697,258 HIGHLY CORROSION RESISTANT MARAGING STAINLESS STEEL Stephen Floreen, Suifern, N.Y., assignor to The International Nickel Company, Inc., New York, N.Y. N0 Drawing. Filed Oct. 13, 1969, Ser. No. 865,962 Int. Cl. C22c 39/20 US. Cl. 75-128 B 12 Claims ABSTRACT OF THE DISCLOSURE A maraging stainless steel containing correlated amounts of nickel, chromium, cobalt, silicon and metal from the group consisting of aluminum and titanium, the steel being particularly suitable in cast form since it affords a combination of good strength, toughness and high corrosion resistance and also exhibits excellent foundry characteristics.

Nearly a decade has passed since the original disco-very of the so-designated maraging steels. And while research continues on an extensive basis there is not yet on the commercial scene, at least insofar as I am aware, a low cost, cast stainless maraging steel. Conventional cast versions, however, have long been incommercial production, for example, the cast maraging steel described in U.S. Patent No, 3,132,937. Indeed, significant processing improvements have been attained in respect of such cast steels as evident from the heat treating procedures set forth in U.S. Patent .No. 3,341,372. Nonetheless, such steels are, at least comparatively speaking, relatively costly and, more importantly, do not exhibit stainless characteristics.

Thus, until recently a hiatus has existed in the maraging family of steels, to wit, a cast stainless maraging steel which combines relatively low cost, high corrosion resistance, good foundry characteristics and a tensile strength on the order of, say, at least 150,000 p.s.i., coupled with good toughness. This hiatus, however, has been substantially narrowed in accordance with the invention described in my co-pending' application, Ser. No. 865,969 filed on Oct. 13, 1969, now abandoned. In that application, cast maraging stainless steels are disclosed as containing about to about 12.5% chromium, 7.5% to 11% nickel and specified percentages of other constituents including silicon, titanium, aluminum and carbon.

Notwithstanding the attractive combination of properties offered by the cast stainless steels above described, where the optimum in corrosion resistance is required they are found lacking mainly due to the limitation imposed in respect of the maximum chromium percentage, to wit, 12.5%. This upper level was necessary to avoid the very practical problems which flow from the presence of excess delta ferrite.

I have found, however, that the delta ferrite problem can be greatly minimized, if indeed not completely obviated, by incorporating special amounts of cobalt in cast maraging stainless steels containing relatively high amounts of chromium, the steels in addition containing correlated amounts of nickel, silicon and metal from the group consisting of aluminum and titanium.

Generally speaking, the present invention contemplates cast maraging stainless steels containing, in weight .percent, from 12.5% to not more than 17% chromium, about 3% to 6.5% nickel, about 6% to 12% cobalt, the sum of the nickel plus cobalt being not less than 11% and not greater than about 15%, about 1.5% to 3% silicon, a small but effective amount, e.g., 0.01%, of metal from the group consisting of aluminum and titanium, up to about 1% manganese, up to about 0.05% carbon and the balance essentially iron. Elements such as phosphorus, sulfur, oxygen and nitrogen should be kept at low levels consistent with good commercial steelmaking practice.

In carrying the invention into practice, should the chromium content exceed about 17% there is an unnecessary risk with respect to loss of certain mechanical characteristics. A chromium level of about 14% to 16% is quite satisfactory.

Nickel is necessary to impart toughness as well as to inhibit formation of delta ferrite. And, in this respect at least 3%, advantageously at least 4%, nickel should be present. But, nickel does depress M temperature as do other elements including chromium. However, it has been found that cobalt tends to stabilize the austenite and to inhibit the formation of delta ferrite that might otherwise be expected to flow from the higher chromium con tents. Too, it has been determined that cobalt does not depress the M temperature to the extent of either nickel or chromium. This, therefore, enables the use of less nickel in counteracting the adverse influence of delta ferrite while at the same time restricting the drop in M temperature. It might also be added that this is achieved while maintaining adequate resistance to impact.

With regard to silicon, it should not fall below about 1.5%; otherwise, low tensile strengths ensue. Percentages much above 3% silicon, while imparting strength, detract from tougness, particularly the ability of the steels to absorb impact energy. A silicon range of 1.6% to 2.5%, e.g., 1.8% to 2.3%, is most satisfactory.

In the absence of either or both aluminum and titanium mechanical properties, particularly in respect to air melted steels, are deleteriously affected. These constituents are deemed to tie up interstitials (carbon, nitrogen) and such elements as oxygen, the presence of which might otherwise bring about an undesirable loss in toughness. Only a small amount, e.g., 0.01% or 0.02%, of either or both of these constituents need be present and it is not necessary that either exceed 0.1% or 0.2% or that the total amount exceed about 0.3%. Preferably, the steels contain not more than about 0.2% of aluminum and/or titanium. In aiming for optimum properties from 0.02% to 0.07% of each of these elements has been found to afford excellent results.

For the purpose of giving those skilled in the art a better understanding of the invention the following illustrative data are given.

A series of 30 pound air-induction melts utilizing electrolytic grade metals as starting materials was prepared. The furnace was first charged with iron and nickel together with about 0.05% carbon (carbon boil) for the deoxidation. Thereafter, about 0.1% each of aluminum and titanium was added followed by the silicon addition. The heats were cast in dry sand double keel block molds, the leg of each keel being 1" x 1%" x 7 in length. Standard tensile A") and Charpy V-Notch impact specimens were machined from the keel block and were therafter solution annealed about one hour at 1900 F., air cooled 3 and maraged at about 850 F. for about 3 hours. The results of these tests are reported in Tables I and II.

TABLE I Percent Ni Cr S1 0 Al Ti Fe 3. 9 15. 3 3. 1 1. 81 018 03 06 Bal. 3. 9 15. 6. 0 1. 92 017 04 07 Bal. 2. 1 17. 5 ll. 8 1. 99 013 .03 05 Bal. 6.0 18. 9 6. 0 1. 93 018 03 05 Bal. 5. 1 13. 9 8. 9 1. 82 022 02 04 Bal. 4. 0 15. 9 9. 2 1. 86 017 02 05 Bal. 4. 1 15. 6 10. 5 1. 93 010 03 06 Bal. 4. 1 15. 7 7. 4 1. 72 014 02 04 Ba].

No'rE.Bal.=Balance essentially iron plus impurities.

TABLE II Elonga- C Y.S., U.T.S., tion, ft.- Alloy K 5.1. K s.i. percent percent lbs.

l 0.2% ofiset.

As can be discerned from a perusal of the above data, alloys containing a total sum of nickel plus cobalt significantly below 12% (Alloys A and B) manifest a greatly inferior capability to absorb high levels of impact energy. Too, even though the nickel plus cobalt relationship be satisfied, it does not necessarily follow that good results will obtain as can be seen from Alloy C in which the nickel content was at the comparatively low level of 2.1% In contrast to Alloys A through C, Alloys 1 through 5 all exhibited a good combination of mechanical characteristics. A particularly good compositional range is as follows: about 14%: to about 16% chromium, about 4% to 6% nickel about,6% to about 10.5% cobalt, the sum of the nickel plus cobalt being from 11.5% or 12% to about 14%, about 1.8% to 2.3% silicon, up to 0.5% manganese, with the aluminum, titanium and carbon being as above described.

In addition to the foregoing, itshould be added that to eifect the maximum in transformation of austenite to martensite upon cooling from hot working or solution treating the steels can also be subjected to a cold treatment, e.g., as by refrigeration down to temperatures .of at least or as low as minus 100 F., and/or by cold working. However, to minimize additional processing operations and in striving for an optimum combination of mechanical characteristics together with a high M temperature, the nickel, cobalt and chromium should be correlated such that the sum of the nickel plus cobalt is from 11.5% to 14% and the total sum of the nickel plus cobalt plus chromium is not more than 28%.

Various of the alloys set forth in Table I were also exposed to different corrosion tests. In this regard, panels of Alloys 1 through 5 (about 1" x 4" x Ma" thick) were exposed in the maraged condition to the wellaknown Salt Spray (Fog) Test in accordance with ASTM designation Bl17-6l. Upon examination of the panels, they were re-tested in the more severe Copper-Accelerated Acetic Acid-Salt Spray (CASS) Test (ASTM designation B368- 61T). Since both of these tests are generally well-known by those skilled in the art, a detailed description is omitted but is described in'Specifications and Tests for Electrodeposited Metallic Coatings. ASTM Philadelphia 1961, 3rd Ed. The results of these tests indicated that the alloys were superior to AISI Type 410 stainless with the alloys containing 15% to 16% chromium being comparable to AISI Type 430.

In addition to the Fog and CASS tests, panels of Alloys 2 and 3 were exposed in the maraged condition for a period of 6 months at the well-known Kure Beachtest site to determine the resistance of the alloys to a marine atmosphere environment. This test confirmed that the alloys containing higher percentages of chromium afforded the greater resistance to corrosive attack.

In processing steels in accordance herewith, usual maraging technology is quite satisfactory. The foundry characteristics of Alloys 1 through 5, Tablev I, were found to be good. As to solution treating, the subject steels can be solution annealed at temperatures as high as 2100" F. but the temperature need not exceed about 1900 F. Though this temperature might be somewhat below that used for conventional cast maraging steels, it nonetheless somewhat simplifies the heat treating operation in the sense that the lower temperatures are more practical for commercial operation. The aging treatment should be conducted over a temperature range of about 800 F. to 1000 F. the preferred range being on the order of 850 F. to 950 F.

The steels of the present invention can be utilized for such applications as wearing rings, compressor wheels, corrosion resistant gears, high pressure valves, propellers, components for power plant pumps, including impellers, stage pieces, diffusers, etc., and for applications generally requiring steels which manifest a good combination of corrosion resistance, strength and toughness. Though the steels are primarily intended for use in the cast form, they can also be used in the wrought condition.

It is to be understood that the expressions balance or balance essentially used in referring to the iron content of the steels in accordance herewith, are not intended to exclude the presence of other elements, e.g., deoxidizing and cleansing constituents, and impurities normally asso-.

be within the purview and scope of the invention and appended claims.

I claim:

1. A maraging stainless steel containing from 12.5% to 17% chromium, about 3% to about 6.5% nickel, about 6% to 12% cobalt, the sum of the nickel plus cobalt being not less than 11% and not greater than about 15%, about 1.5% to about 3% silicon, metal from the group consisting of titanium and aluminum in a small but effective amount sufiicient to enhance the toughness of the steel, the titanium and aluminum not exceeding about 0.2% each, up to about 1% manganese, up to 0.05% carbon and the balance iron.

2. A steel in accordance withclaim 1 in the cast condition.

3. A steel in accordance with claim 2 containing from about 14% to 16% chromium.

4. A steel in accordance with claim 2 containing at least 4% nickel.

5. A steel in accordance with claim 2 containing 1.6% to 2.5% silicon.

6. A steel in accordance with claim 2 in which the sum of the cobalt plus chromium does not exceed 14% and the sum of the nickel plus cobalt plus chromium does not exceed about 28%.

7. A steel in accordance with claim). in which aluminum is present in an amount of from about 0.01% to about 0.1%.

8. A steel in accordance with claim 2 in which titanium (iffgesent in an amount of from about 0.01% to about 9. A steel in accordance with'claim 2 containing about 0.02% to 0.07% each of aluminum and titanium.

10. A steel in accordance with claim 2 containing about 14% to 16% chromium, about 4% to 6% nickel, about 6% to about 10.5% cobalt, about 1.8% to 2.3% silicon, up to 0.03% carbon, up to about 0.5% manganese and about 0.01% to 0.1% each of titanium and aluminum.

11. A steel in accordance with claim 10 containing 0.02% to 0.07% each of aluminum and titanium.

12. A steel in accordance with claim 10 in which the sum of the nickel plus cobalt does not exceed 14% and in which the sum of the nickel plus cobalt plus chromium does not exceed 28%.

References Cited UNITED STATES PATENTS Perry 75128 R Hammond 75-128 R Kasa'k 75-126 R Binder 75-l28 B lHarris 75--126 R Brady 75-128 R US. Cl. X.R. 

